Correlation and age of the Upper Jurassic Morrison Formation from magnetostratigraphic analysis

Abstract

The magnetostratigraphy of the Morrison Formation of east-central New Mexico resembles that of three western Colorado sections. Magnetic polarity and lithology agree among the sections, indicating the correlation potential of magnetostratigraphy in this lithologically complex formation. Both magnetostratigraphy, lithology, and paleopoles divide the formation into two parts. The lower sandstone-dominated portion was deposited during a time of relatively equal lengths normal and reversed polarity intervals, whereas the upper mudstone (Brushy Basin) portion was deposited during predominantly reversed polarity with interspersed short normal intervals. The four magnetostratigraphic sequences yield a composite Morrison Formation magnetostratigraphy. Upper Morrison polarity correlates well with the polarity pattern of the marine magnetic anomaly M-sequence and indicates that the Brushy Basin Member is Kimmeridgian and earliest Tithonian in age. The correlation suggests that lower part of the formation is at least Oxfordian in age. Two pole positions were obtained from the New Mexico Morrison Formation. The lower sandstone-dominated part of the formation gives a different paleopole from the mostly mudstone upper portion of the formation. This difference is quite similar to observation of two pole positions in the Colorado Morrison Formation. The similarity between New Mexico and Colorado indicates that the base level change within the formation, the change from sandstone and mudstone to nearly purely mudstone deposition, was approximately synchronous in the two areas. The presence of two different paleopoles suggests an hiatus in the formation. The hiatus in sedimentation coincides with the base level change, and the difference in paleopoles suggests that sufficient time elapsed for continental motion to occur between deposition of the lower members of the Morrison Formation and the Brushy Basin Member above them.